Method of applying a coating to a perforated substrate

ABSTRACT

The present invention provides a method of applying a coating to a perforated substrate, the method comprising: (a) providing a perforated substrate having a substrate first surface, a substrate second surface and one or more perforations traversing the substrate from the first surface to the second surface; (b) bringing the substrate first surface into contact with a perforation blocking solid; (c) applying a metallic thermal spray coating composition to the substrate second surface using a thermal spray technique; and (d) separating the perforation blocking solid from the substrate first surface to provide a substrate having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating. Among its many other uses, the method is also useful for applying thermal spray coatings to the interior surface of valve cages used in flow control valves.

BACKGROUND

The present invention relates generally to applying a coating aperforated substrate. In particular, the invention relates to methodsand techniques of applying a metallic thermal spray coating to aperforated substrate, and wherein the perforations are not occluded bythe metallic thermal spray coating.

In numerous areas of science and commerce it is desirable to apply ametallic thermal spray coating to a perforated surface in such a waythat the surface is rendered more resistant to erosive and corrosiveagents, while at the same time not occluding the perforations. Variousschemes for achieving this desirable outcome have been advanced and/oremployed. These schemes include, for example, coating the substrateprior to the creation of the perforations, and thereafter machining theperforations through the metallic thermal spray coating and thesubstrate. Such machining can reduce the overall structural integrity ofless robust metallic thermal spray coatings. In the case of very hardmetallic thermal spray coatings, such machining can damage the machiningtools themselves and be time intensive. Other schemes include fillingthe perforations with a readily removable filler to which the metallicthermal spray coating has a low adhesive affinity. This necessitatesidentification of the perforation-filling material, its introductioninto the perforations of the perforated substrate, and the removal ofsuch perforation-filling material from the perforations following theapplication of the metallic thermal spray coating to the substrate. Theconsiderable expenditure of human ingenuity and effort in this areanotwithstanding, further improvements are both desired and needed,particularly in the field of control valves in which a simple andreliable method of applying a metallic thermal spray coating to aperforated substrate would be especially prized.

The present invention provides a simple and surprisingly effectivemethod of applying a metallic thermal spray coating to a perforatedsubstrate.

BRIEF DESCRIPTION

In one embodiment, the present invention provides a method of applying acoating to a perforated substrate, the method comprising: (a) providinga perforated substrate having a substrate first surface, a substratesecond surface and one or more perforations traversing the substratefrom the first surface to the second surface; (b) bringing the substratefirst surface into contact with a perforation blocking solid; (c)applying a metallic thermal spray coating composition to the substratesecond surface using a thermal spray technique; and (d) separating theperforation blocking solid from the substrate first surface to provide asubstrate having a metallic thermal spray coating disposed upon thesubstrate second surface and wherein the perforations are not occludedby the metallic thermal spray coating.

In an alternate embodiment, the present invention provides a method ofapplying a coating to a perforated article, the method comprising: (a)providing a perforated article having a first surface, a second surfaceand one or more perforations traversing the article from the firstsurface to the second surface; (b) bringing the first surface intocontact with a perforation blocking solid; (c) applying a metallicthermal spray coating composition to the second surface using a thermalspray technique; and (d) separating the perforation blocking solid fromthe first surface to provide an article having a metallic thermal spraycoating disposed upon the second surface and wherein the perforationsare not occluded by the metallic thermal spray coating.

In yet another embodiment, the present invention provides a method ofapplying a coating to a perforated cylindrical metal article, the methodcomprising: (a) providing a perforated cylindrical metal article havingan outer first surface and an inner second surface and one or moreperforations traversing the substrate from the first surface to thesecond surface; (b) wrapping the outer first surface of the perforatedcylindrical metal article with a perforation blocking solid; (c)applying a metallic thermal spray coating composition to the innersecond surface using a thermal spray technique; and (d) separating theperforation blocking solid from the first surface to provide acylindrical metal article having a metallic thermal spray coatingdisposed upon the substrate second surface and wherein the perforationsare not occluded by the metallic thermal spray coating.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Various features, aspects, and advantages of the present invention willbecome better understood when the following detailed description is readwith reference to the accompanying drawings in which like characters mayrepresent like parts throughout the drawings. Unless otherwiseindicated, the drawings provided herein are meant to illustrate keyinventive features of the invention. These key inventive features arebelieved to be applicable in a wide variety of systems which comprisingone or more embodiments of the invention. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the invention.

FIG. 1 illustrates a valve cage and valve plug combination usedaccording to one or more embodiments of the present invention.

FIG. 2 illustrates a method used according to one or more embodiments ofthe present invention.

FIG. 3 illustrates a method step used according to one or moreembodiments of the present invention and an illustration of one theoryof operability of the present invention.

DETAILED DESCRIPTION

In the following specification and the claims, which follow, referencewill be made to a number of terms, which shall be defined to have thefollowing meanings.

The singular forms “a”, “an”, and “the” include plural referents unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about” and “substantially”, are not to be limited tothe precise value specified. In at least some instances, theapproximating language may correspond to the precision of an instrumentfor measuring the value. Here and throughout the specification andclaims, range limitations may be combined and/or interchanged, suchranges are identified and include all the sub-ranges contained thereinunless context or language indicates otherwise.

In one or more embodiments, the present invention provides methods andtechniques for applying a metallic thermal spray coating to one or moresurfaces of a perforated substrate, wherein the perforations are notoccluded by the metallic thermal spray coating being applied to thesubstrate. The method depends upon the inventors' rather remarkablefinding if that a substrate traversed by one or more perforationsextending from a first surface of the substrate to a second surface ofthe substrate is contacted with a metallic thermal spray at the firstsurface, the simple expedient of blocking the outlets of theperforations at the first surface results in the perforation openings atthe second surface being remarkably resistant to becoming occluded bythe metallic thermal spray coating as it builds upon on the secondsurface of the substrate. This disclosure provides details of thisdiscovery, guidance on how it may be practiced and exploited, and dataillustrating the utility of the discovery in the field of control valvemanufacture.

The perforated substrate may be any substrate containing perforationsextending from a substrate first surface, through the substrate and to asubstrate second surface and susceptible to being coated using a thermalspray technique, such as the HVAF (High Velocity Air Fuel), AC-HVAF(Activated Combustion High Velocity Air Fuel) and HVOF (High VelocityOxygen Fuel) thermal spray techniques. By being susceptible to beingcoated by a thermal spray technique, it is meant that the substrate isnot destroyed or rendered useless by the coating technique. Typically,the substrate is a metallic substrate which can withstand the hightemperatures inherent in thermal spray techniques. In one embodiment,the perforated substrate is essentially metallic. In an alternateembodiment, the perforated substrate may be a non-metallic material, forexample a ceramic or a composite, which is stable under the conditionsof the thermal spray technique. A wide variety of substrate coolingtechniques are known to those of ordinary skill in the art, and suchcooling techniques may be used to mitigate the sometimes negativeeffects of substrate temperature build-up during the application of themetallic thermal spray coating to the substrate.

In one or more embodiments the substrate is a sheet of metal defining asubstrate first surface and a substrate second surface and containing aplurality of perforations extending from the first surface to the secondsurface. In one embodiment the first surface is substantially parallelto the second surface, meaning that the first surface and the secondsurface define two substantially parallel planes. When this condition ismet, the substrate first surface and the substrate second surface aresaid to be substantially parallel surfaces. In one embodiment the firstsurface is substantially non-parallel to the second surface, meaningthat the first surface and the second surface do not define twosubstantially parallel planes. When this condition is met, the substratefirst surface and the substrate second surface are said to besubstantially non-parallel surfaces. A flat, sheet-like perforatedsubstrate in which the first surface and the second surface haveessentially the same surface area and define substantially parallelplanes is an example of the perforated substrate wherein the substratefirst surface and the substrate second surface are substantiallyparallel surfaces. See for example perforated substrate 530 in FIG. 3 ofthis disclosure. If the same sheet-like structure were made to bendand/or twist into a structure having curved surfaces, the bent and/ortwisted structure would qualify as a perforated substrate in which thesubstrate first surface and the substrate second surface aresubstantially non-parallel surfaces. The foregoing discussion ofsubstantially parallel and substantially non-parallel substrate firstsurfaces and substrate second surfaces ignores edge surfaces. In oneembodiment, the perforated substrate is a perforated cylindrical metalarticle such as a cage for a flow control valve.

As noted, blocking the outlets of the perforations at the substratefirst surface results in being able to apply a metallic thermal spraycoating to the substrate second surface such that the perforations arenot occluded by the metallic thermal spray coating as the coating isdeposited on the substrate second surface. This means that the openingsof the perforations at the substrate second surface remain substantiallyunchanged as the substrate second surface is transformed from an initialuncoated stage to a coated surface. This effect is demonstratedexperimentally herein (See Experimental Part). The inventors themselveshave proposed various theories to account for this effect and while notwishing to be bound by any particular theory, it is possible thatblocking the outlets of the perforations at the substrate first surfaceresults in the creation of a relatively high local pressure within theperforations during the application of the metallic thermal spraycoating as gas molecules present between the thermal spray and thesubstrate second surface are forced by the moving thermal spray into theperforations. As the thermal spray is applied in a zone containing anopening of a perforation at the substrate second surface, thisrelatively higher local pressure within the perforation creates a biasaway from the opening and directs the thermal spray particles onto thesubstrate second surface adjacent to the opening. An alternative theory,illustrated in FIG. 3, posits that incident thermal spray particlessimply ricochet off the closed end of the perforation.

Blocking the outlets of the perforations at the substrate first surfacecan be effected simply by bringing the substrate first surface intocontact with a perforation blocking solid which conforms to thesubstrate first surface. In the case of a planar substrate, theperforation blocking solid can be a strip of sheet metal of sufficientbreadth to block the outlets of the perforations at the substrate firstsurface. In another embodiment, the perforation blocking solid isnon-planar and conforms to a non-planar substrate first surface. Forexample, in one embodiment, the substrate first surface is the outersurface of a perforated cylinder and the perforation blocking solid is ametal sleeve which conforms to the substrate first surface. Those ofordinary skill in the art will understand that the perforation blockingsolid must be in close physical contact with the perforation outlet atthe substrate first surface, and that in certain embodiments theperforation blocking solid may comprise surface features whichcomplement and couple with the perforation outlets at the substratefirst surface. It should be stressed, however, that it is unnecessary tocompletely fill the perforation volume in order to prevent occlusion ofthe perforation at the substrate second surface. Simply blocking theperforation outlet at the substrate first surface is sufficient.

As noted, the perforation blocking solid conforms to the substrate firstsurface and preferably has a low surface roughness such that if oneattempted to apply a metallic thermal spray coating using a standardthermal spray technique, adhesion of the coating to the perforationblocking solid would be poor. In one embodiment, the perforationblocking solid has a surface roughness less than 32 R_(a). In oneembodiment, the perforation blocking solid comprises stainless steel. Inan alternate embodiment, the perforation blocking solid comprisesaluminum. In yet another embodiment, the perforation blocking solidcomprises chromium.

In contrast to the perforation blocking solid, the substrate secondsurface to which the metallic thermal spray coating is to applied shouldhave a suitable surface roughness to promote adhesion of the metallicthermal spray coating. Various techniques are known to those of ordinaryskill in the art for increasing or decreasing the surface roughness ofan article's surface. In one embodiment, the substrate second surfacehas a surface roughness of about 100 R_(a). In another embodiment, thesubstrate second surface has a surface roughness greater than 100 R_(a).

As noted, following the application of the metallic thermal spraycoating to the substrate second surface, the perforation blocking solidis separated from the substrate first surface to provide a substratehaving a metallic thermal spray coating disposed upon the substratesecond surface and wherein the perforations are not occluded by themetallic thermal spray coating. As used herein, the term “not occluded”means that the cross-sectional area of the opening of the perforation atthe substrate second surface after the application of the metallicthermal spray coating is at least eighty-five (85) percent of thecross-sectional area of the opening of the perforation at the substratesecond surface prior to the application of the metallic thermal spraycoating. In one embodiment, the cross-sectional area of the opening ofthe perforation at the substrate second surface after the application ofthe metallic thermal spray coating is at least ninety (90) percent ofthe cross-sectional area of the opening of the perforation at thesubstrate second surface prior to the application of the metallicthermal spray coating. In another embodiment, the cross-sectional areaof the opening of the perforation at the substrate second surface afterthe application of the metallic thermal spray coating is at leastninety-five (95) percent of the cross-sectional area of the opening ofthe perforation at the substrate second surface prior to the applicationof the metallic thermal spray coating.

The perforations may be of uniform or non-uniform size within theperforated substrate. In one embodiment, the perforated substratecomprises perforations of uniform size, meaning that each perforationhas substantially the same shape and dimensions as all otherperforations in the perforated substrate. In another embodiment, theperforated substrate comprises perforations of non-uniform size, meaningthat at least two perforations differ significantly in their size and/orshape.

As noted, the perforations traverse the perforated substrate from aperforation opening at the substrate second surface to a perforationoutlet at the substrate first surface. In one embodiment, theperforations may be substantially orthogonal to the substrate firstsurface and substrate second surface See, for example, perforatedsubstrate 530 in FIG. 3. In another embodiment, the perforations may besubstantially non-orthogonal to the substrate first surface and thesubstrate second surface. In an alternate embodiment, the perforatedsubstrate comprises perforations some of which are substantiallyorthogonal to the substrate first surface and substrate second surfaceand some of which are substantially non-orthogonal to the substratefirst surface and the substrate second surface. In one or moreembodiments, the perforations cylindrical in shape and have averagediameter in a range from about 0.1 mm to about 10 mm.

The perforated substrate used according to one or more embodiments ofthe invention may be prepared from an un-perforated substrate by variousmeans known to those of ordinary skill in the art, for example machiningand laser ablation techniques. In one embodiment, the perforatedsubstrate is prepared from an un-perforated substrate by drillingperforations from an inlet at the substrate second surface to an outletat the substrate first surface.

The metallic thermal spray coating composition employed according to theinvention may be any metallic coating composition suitable forapplication by a thermal spray technique. In one embodiment, the thermalspray coating composition employed comprises tungsten carbide, cobaltand chromium and is characterized by an average particle size (prior toapplication) in a range from about 1 micron to about 100 microns.Additional suitable metallic thermal spray coating compositions areknown to those of ordinary skill in the art and include tungstencarbide/nickel, tungsten carbide/nickel-chromium, chromiumcarbide/nickel-chromium, and cobalt/chromium/carbon alloys.

Those of ordinary skill in the art will understand that the term“perforated substrate” may be applied to perforated articles, aperforated article being a subset of the set which includes allperforated substrates. The distinction between a perforated substrateand a perforated article is that the term “article” refers to (1)substrates which can be recognized as independent components of a systemor device, or (2) substrates capable of independently performing afunction. According to this definition, all articles are substrates, butnot every substrate qualifies as an article.

As noted, in one embodiment, the present invention provides method ofapplying a coating to a perforated article, the method comprising: (a)providing a perforated article having a first surface, a second surfaceand one or more perforations traversing the article from the firstsurface to the second surface; (b) bringing the first surface intocontact with a perforation blocking solid; (c) applying a metallicthermal spray coating composition to the second surface using a thermalspray technique; and (d) separating the perforation blocking solid fromthe first surface to provide an article having a metallic thermal spraycoating disposed upon the second surface and wherein the perforationsare not occluded by the metallic thermal spray coating.

In one embodiment, the article is a cage for a flow control valve. Flowcontrol valves are exemplified by flow control valves known to those ofordinary skill in the art and include, for example the 40005 series(e.g. Models 41305, 41505, 41605 and 41905) of flow control valvesavailable from Dresser Flow Control an affiliate of the General ElectricCompany. As used herein a valve cage for a flow control valve may incertain embodiments qualify as a perforated cylindrical metal article(See for example FIGS. 1 and 2 of this disclosure.)

In one embodiment, the present invention provides a method coating acage (at times herein referred to as a “valve cage”) for a flow controlvalve with an erosion resistant metallic thermal spray coating whereinthe coating can be applied according to one or more embodiments of thepresent invention and without occluding the perforations (holes) presentin the valve cage structure.

Turning now to the figures, FIG. 1 illustrates a valve cage and valveplug combination shown in cutaway view 300 and three dimensional view302 the performance of which may be enhanced by applying metallicthermal spray coating on the interior surface 320 of valve cage 212. Thevalve cage and valve plug combination shown in FIG. 1 is containedwithin a control valve coupled to a working fluid inlet conduit and aworking fluid outlet conduit. As shown in the figure valve cage 212consists of an upper un-perforated portion 420 (FIG. 2) and a lowerperforated portion 410 (FIG. 2). Perforations 213 traverse the valvecage from the inner surface 320 to the outer surface of the valve cage.Valve plug 310 is disposed within a central cavity defined by the valvecage and is sized such that it may move freely within the valve cagecavity and yet free space between the main body of the valve plug andthe interior surface of the valve cage is minimized in order to limitleakage of a working fluid through said free space and into othercomponents of the valve. The valve is opened by moving the valve plug inthe cavity defined by the valve cage so as to expose one or more of thecage perforations to the working fluid which may then enter and passthrough the perforation and into a suitable exit conduit. In oneembodiment, the working fluid may enter the interior cavity of the valvecage from a fluid inlet conduit (not shown) connected to the lower endof the valve and exit the valve through perforations 213 which emptyinto a working fluid outlet conduit (not shown). Such valves aretypically not self-actuating, and an active control mechanism istypically employed to open and close the valve. Thus a drive rod (notshown) may be coupled with valve plug stem 330 to open and close thevalve to the extent required to achieve a desired flow rate of theworking fluid across the valve.

The foregoing discussion of control valve mechanics highlights the needfor erosion and corrosion resistant coatings, especially at theinterface between the valve plug and the valve cage. The working fluidfrequently contains hard particles such as sand and metallic particleswhich can erode the surfaces of the valve plug and the surface of thevalve cage. In the embodiment illustrated in FIG. 1 this interfacebetween the valve plug and the valve cage includes the interior surface320 of the perforated portion 410 of the valve cage 212. While manymetallic thermal spray coatings are known to enhance surface hardness,wear resistance and resistance to corrosive chemicals, the applicationof such metallic thermal spray coatings to the surfaces of perforatedarticles without occluding the perforations has been notoriouslydifficult.

FIG. 2 illustrates an embodiment of the present invention which is amethod 400 of applying a metallic thermal spray coating to the innersurface of a control valve cage 212. In a first method step 440, a sheetof flexible metal 430 is wrapped around the outer surface (substratefirst surface) of the perforated portion 410 of the valve cage in such away that the surface of the sheet of flexible metal conforms to theouter surface of the valve cage. The sheet of flexible metal is held inplace by securing clamps 450. In a second method step 442 a metallicthermal spray coating is applied to the inside surface 320 (substratesecond surface) of the valve cage using a thermal spray technique. Whenthe desired thickness of the coating has been achieved, the sheet offlexible metal 430 may be removed in a third method step 444 to providethe valve cage comprising an metallic thermal spray coating on theinterior surface of the valve cage and wherein the perforations are notoccluded by the metallic thermal spray coating. In alternateembodiments, a metal sleeve closely conforming to the dimensions of theouter surface of the perforated portion of the valve cage may be usedinstead of a sheet of flexible metal.

Referring now to FIG. 3 the figure represents the application of ametallic thermal spray coating onto a substrate second surface 535 of aperforated substrate 530. A robot controlled thermal spray nozzle 510moves above the substrate second surface 535 of the perforated substrate530 along axes of motion 512 while directing thermal spray 520 towardthe perforated substrate. The figure shows a perforation blocking solid540 conforming to a substrate first surface 536 and serving to block thelower end of the perforations. The figure also illustrates one theory ofhow the present invention works. Under this theory, incident thermalspray particles 522 collide with the surface of the perforation blockingsolid 545 at the bottom of perforation in ricochet events 550, producingthereby a stream of ricocheting particles 524 exiting the perforation.Although, not wishing to bound by this or any other theory ofoperability, this model accounts for the limited occlusion of theperforations observed, since the ricocheting thermal spray particles 524exiting the perforation will tend to scour the edges of the perforationat its end adjacent to substrate second surface 535. It should be notedthat incident thermal spray particles impacting substrate second surface535 will tend adhere to that surface, since the substrate is selectedsuch that it has a high affinity for metallic thermal spray particles.In addition, surface treatments known to those of ordinary skill in theart may be employed to enhance the affinity of a substrate secondsurface such as 535 for thermal spray particles such as 522.

Experimental Part General Methods

The thermal spray equipment and metallic thermal spray coating precursorcompositions employed in the application of metallic thermal spraycoatings are commercially available from suppliers such as Kermetico,Inc.; Sulzer Metco, Inc.; and Praxair, Inc. Commercially availablecylindrical valve cages having a plurality of substantially roundperforations extending from the interior surface of the valve cage tothe exterior surface of the valve cage were obtained from Dresser FlowControl, Inc. All of the perforations had a diameter of about 0.12inches. The valve cage had a perforated portion extending from a firstend of the valve cage to approximately the midpoint along the length ofthe valve cage. In addition, the valve cage had an un-perforated portionextending from a second end of the valve cage to approximately themidpoint along the length of the valve cage.

A recessed zone approximately 0.010 inch deep and several centimeterswide (also referred to as “the machined zone”) was machined into theinside surface of the valve cage near the interface between theperforated portion and the un-perforated portion of the valve cage andextending several centimeters into the un-perforated portion of thevalve cage to compensate for additional metallic thermal spray coatingthickness in the region of the recessed zone that resulted from thethermal spray protocol used. In this thermal spray protocol, the thermalspray gun was, of necessity, positioned outside of the interior cavityof the valve cage but along the central axis of the valve cage duringthe application of the metallic thermal spray coating. It should benoted that the thermal spray gun apparatus was too large to be insertedinto the interior cavity of the valve cages studied. The metallicthermal spray coating was applied to the interior surface of the valvecage in two steps. In a first step, the metallic thermal spray coatingwas applied through a first end of the valve cage continuously onto thevalve cage interior surface from the portion of the valve cage interiorsurface closest to first end, and up to a point just beyond the machinedzone along the inside diameter of the valve cage. In a second step, themetallic thermal spray coating was applied through a second end of thevalve cage (opposite the first end) continuously onto the valve cageinterior surface from the portion of the valve cage interior surfaceclosest to second end, and up to a point just beyond the machined zonealong the inside diameter of the valve cage. As such, the insidediameter of the valve cage in the region of the machined zone of thevalve cage was susceptible to being coated from each end of the valvecage, and the resultant metallic thermal spray coating was approximatelytwice as thick on the interior surface of the valve cage in the regionof the machined zone relative to other regions of the interior surfaceof the valve cage. The machined zone along the interior diameter wasincluded to ensure that this additional thickness did not interfere withthe performance of the valve cage when in combination with a movablevalve plug. As an additional precaution against further overlap ofmetallic thermal spray coatings applied from either end of the valvecage, a removable metallic mask cylinder was inserted into the portionof the valve cage interior cavity not being subjected to the thermalspray technique at a given time, such that the interior surface of thevalve cage as measured from a first end of the valve cage opposite asecond end of the valve cage through which the thermal spray was beingapplied, and up to the edge of the machined zone closest first end ofthe valve cage was covered by the removable metallic mask cylinder andprotected from the incident thermal spray.

The interior surface of the valve cage, including the machined zone, wasgrit blasted prior to the application of the metallic thermal spraycoating. Following grit blasting the exterior surface of the perforatedportion of the valve cage was brought into contact with a piece offlexible rolled stainless steel sheet metal of appropriate length andbreadth to cover all of the valve cage perforations and was estimated tohave a surface roughness of about 16 R_(a). The sheet metal was securedby hose clamps which assured close contact between the perforationblocking sheet metal and the openings of the perforations at the valvecage outer surface.

The thermal spray gun was oriented such that the thermal spray wasdirected towards the interior surface of the valve cage at an angle ofincidence of about 45 degrees. The valve cage was rotated around itsaxis while the thermal spray gun applied the metallic thermal spraycoating and moved axially at a translation speed of about 1000millimeters per second through a continuous series of axial positionsover a distance of about 75 millimeters which maintained a workingdistance between the thermal spray gun and the portion of the interiorsurface being coated at any given time of about 15 inches. After itsclosest approach to the work piece, the thermal spray gun reverseddirections and returned to its original position while continuing toapply the metallic thermal spray coating. The process was repeated,typically through about 18 such cycles (also referred to at times hereinas “reps”), until the metallic thermal spray coating disposed upon valvecage interior surface was judged to be of sufficient thickness,typically 9 to 10 mils as measured using a micrometer at multiple pointsalong the valve cage walls. This provided a valve cage in which slightlymore than half of the interior surface (the first portion) was coatedwith the metallic thermal spray coating. The remaining uncoated portionof the interior surface of the valve cage was coated using the sametechnique used to coat the first portion except ingress of the thermalspray was from the opposite end of the valve cage.

In one or more experiments a cermet powder was used as the metallicthermal spray coating precursor and consisted of tungsten carbide (WC)particles that were primarily less than 1 micron in size together with acobalt-chromium (CoCr) matrix. The powder composition was 84 weightpercent WC, 10 weight percent Co and 4 weight percent Cr. Other suitablemetallic thermal spray coating precursor include compositions comprisingabout 85 weight percent WC and about 15 weight percent Co; compositionscomprising about 85 weight percent WC and about 15 weight percent Ni;compositions comprising about 80 weight percent Cr₃C₂ and about 20weight percent Ni20Cr alloy; and compositions comprising about 50 weightpercent Cr₃C₂ and about 50 weight percent Ni 20Cr alloy.

EXAMPLES 1-2 AND COMPARATIVE EXAMPLES 1-2

Data are gathered in Table 1 below which demonstrate the effectivenessof the method of the present invention at prevention of perforationocclusion (closure) during the application of a metallic thermal spraycoating to the interior surface of valve cages. Four control valvecages; two 5.25 inch inner diameter valve cages (Dresser) and two 3.25inch inner diameter valve cages (Dresser) were used in Example 1 andComparative Example 1, and Example 2 and Comparative Example 2respectively. Each valve cage was prepared for thermal spray treatmentas described in the General Methods section. The difference between theExamples and the Comparative Examples being the absence of theperforation blocking solid in the Comparative Examples. Grit blasting ofthe interior surface of each of the valve cages was carried out using 60mesh alumina at 80 psi. Following grit blasting, one of each size of thevalve cages was wrapped with a flexible stainless steel sheet in contactwith the entire outer surface of the perforated portion of the valvecage such that all perforation outlets (hole outlets) were blocked.These wrapped valve cages were used in Examples 1 and 2, and were coatedusing the thermal spray technique described in the General Methodssection using a dry metallic thermal spray coating precursor compositioncomprising WCCoCr (WC760) which was applied as received from acommercial supplier (PRAXAIR). Comparative Examples 1 and 2 wereessentially identical to Examples 1 and 2, with the exception that theperforations were not blocked in any way during the application of themetallic thermal spray coating to the interior surfaces of the two valvecages used in the Comparative Examples.

TABLE 1 cage inner R_(a) Starting After coating diameter thickness(micro- hole size hole size Example (inches) (mils)* inch) (mils)(mils)† Example 1 5.25 11 121 113 Comparative 5.25 11 121 98 Example 1Example 2 3.25 9.95 121 110 Comparative 3.25 9.95 121 97 Example 2*Measured with a micrometer at least three locations on the valve cage.†As determined using a set of pin gages.

The data in Table 1 compare the valve cage hole sizes before and aftercoating. Results for Examples 1-2 when compared Comparative Examples 1-2demonstrate that when valve cage perforations (holes) were in blocked atthe outer surface of the valve cage, the perforations showedsignificantly less occlusion than unblocked perforations (ComparativeExamples 1 and 2) following the application of the metallic thermalspray coating to the interior surface of the valve cage. Pin gauges wereused to measure hole sizes and the size of each of the hundreds of holesdefined by the valve cage was measured.

The foregoing examples are merely illustrative, serving to illustrateonly some of the features of the invention. The appended claims areintended to claim the invention as broadly as it has been conceived andthe examples herein presented are illustrative of selected embodimentsfrom a manifold of all possible embodiments. Accordingly, it isApplicants' intention that the appended claims are not to be limited bythe choice of examples utilized to illustrate features of the presentinvention. As used in the claims, the word “comprises” and itsgrammatical variants logically also subtend and include phrases ofvarying and differing extent such as for example, but not limitedthereto, “consisting essentially of” and “consisting of.” Wherenecessary, ranges have been supplied, those ranges are inclusive of allsub-ranges there between. It is to be expected that variations in theseranges will suggest themselves to a practitioner having ordinary skillin the art and where not already dedicated to the public, thosevariations should where possible be construed to be covered by theappended claims. It is also anticipated that advances in science andtechnology will make equivalents and substitutions possible that are notnow contemplated by reason of the imprecision of language and thesevariations should also be construed where possible to be covered by theappended claims.

What is claimed is:
 1. A method of applying a coating to a perforatedsubstrate, the method comprising: (a) providing a perforated substratehaving a substrate first surface, a substrate second surface and one ormore perforations traversing the substrate from the first surface to thesecond surface; (b) bringing the substrate first surface into contactwith a perforation blocking solid; (c) applying a metallic thermal spraycoating composition to the substrate second surface using a thermalspray technique; and (d) separating the perforation blocking solid fromthe substrate first surface to provide a substrate having a metallicthermal spray coating disposed upon the substrate second surface andwherein the perforations are not occluded by the metallic thermal spraycoating.
 2. The method according to claim 1, wherein the perforatedsubstrate is a metallic substrate.
 3. The method according to claim 1,wherein the substrate first surface and the substrate second surface aresubstantially parallel surfaces.
 4. The method according to claim 1,wherein the substrate first surface and the substrate second surface aresubstantially non-parallel surfaces.
 5. The method according to claim 1,wherein the perforations are substantially orthogonal to the substratefirst surface and the substrate second surface.
 6. The method accordingto claim 1, wherein the perforations are substantially non-orthogonal tothe substrate first surface and the substrate second surface.
 7. Themethod according to claim 1, wherein the perforation blocking solid ismade of sheet metal.
 8. The method according to claim 7, wherein theperforation blocking solid is characterized by a surface roughness ofless than 32 Ra.
 9. The method according to claim 1, wherein the thermalspray technique is an HVAF thermal spray technique.
 10. The methodaccording to claim 1, wherein the thermal spray technique is an HVOFthermal spray technique.
 11. The method according to claim 1, whereinmetallic thermal spray coating composition comprises tungsten carbide,cobalt and chromium and is characterized by an average particle size ina range from about 1 micron to about 100 microns.
 12. A method ofapplying a coating to a perforated article, the method comprising: (a)providing a perforated article having a first surface, a second surfaceand one or more perforations traversing the article from the firstsurface to the second surface; (b) bringing the first surface intocontact with a perforation blocking solid; (c) applying a metallicthermal spray coating composition to the second surface using a thermalspray technique; and (d) separating the perforation blocking solid fromthe first surface to provide an article having a metallic thermal spraycoating disposed upon the second surface and wherein the perforationsare not occluded by the metallic thermal spray coating.
 13. A method ofapplying a coating to a perforated cylindrical metal article, the methodcomprising: (a) providing a perforated cylindrical metal article havingan outer first surface and an inner second surface and one or moreperforations traversing the substrate from the first surface to thesecond surface; (b) wrapping the outer first surface of the perforatedcylindrical metal article with a perforation blocking solid; (c)applying a metallic thermal spray coating composition to the innersecond surface using a thermal spray technique; and (d) separating theperforation blocking solid from the substrate first surface to provide acylindrical metal article having a metallic thermal spray coatingdisposed upon the substrate second surface and wherein the perforationsare not occluded by the metallic thermal spray coating.
 14. The methodaccording to claim 13, wherein the article is a cage for a flow controlvalve.
 15. The method according to claim 13, wherein the perforationsare essentially round and are characterized by a perforation averagediameter in a range from about 0.1 mm to about 10 mm.
 16. The methodaccording to claim 13, wherein the perforation blocking solid is made ofsheet metal.
 17. The method according to claim 16, wherein theperforation blocking solid is characterized by a surface roughness ofless than 32 Ra.
 18. The method according to claim 13, wherein thethermal spray technique is an HVAF thermal spray technique.
 19. Themethod according to claim 13, wherein the thermal spray technique is anHVOF thermal spray technique.
 20. The method according to claim 13,wherein metallic thermal spray coating composition comprises tungstencarbide, cobalt and chromium and is characterized by an average particlesize in a range from about 1 micron to about 100 microns.
 21. The methodaccording to claim 13, wherein metallic thermal spray coatingcomposition comprises chromium carbide, nickel and chromium and ischaracterized by an average particle size in a range from about 1 micronto about 100 microns.